1. Field of the Invention
The present invention relates to a method of coating an optical fiber with a coating composition that can be crosslinked by ultraviolet (UV) radiation. It relates in particular to a method whereby the fiber can be coated immediately after it is drawn from a preform. The method is intended for coating a thin conductor such as an optical fiber, but can also be used to coat any other type of product, in particular products whose fabrication limits the time available for effecting the crosslinking. The invention further encompasses an optical fiber provided with this kind of coating and a telecommunication cable containing this kind of fiber.
2. Description of the Prior Art
It is well known that an optical fiber is intended to transmit light waves. It consists of an optical core surrounded by an optical cladding. The optical core is intended to guide most of the light waves transmitted by the optical fiber. These two components (the optical core and the optical cladding) constitute the optical portion of the fiber, referred to as the bare optical fiber. This portion is highly sensitive to external disturbances, which can lead to deterioration of its transmission properties, and thus to degraded operation of the optical fiber. To protect the bare fiber from such external disturbances, it is well known to coat the optical cladding with a plastics material protection layer, known as the coating, and possibly consisting of several layers. In order to be able to code or recognize the fiber, the external layer of the coating can be colored or covered with a colorizing layer. This kind of coated fiber is usually intended to be included in a telecommunication cable. A plurality of parallel optical fibers can be combined to form a ribbon. In this case, a covering is provided to hold the fibers together.
The role of the coating is to protect the conductor from external mechanical aggression and penetration of moisture and, where necessary, to provide electrical insulation. The coating protects the conductor from mechanical or chemical aggression liable to cause optical transmission attenuation defects. The coating also accounts for the mechanical properties of the conductor; it must in particular absorb microbends and any stresses. Furthermore, it must provide sufficient fire resistance in the event of fire. The coating must adhere well to the support intended for it. In the case of an optical fiber, the physical properties of the coating must be compatible with the fiber drawing conditions, in particular the fiber drawing speed, and with the final use of the fiber.
The solution used at present to produce the protection layers consists in coating the optical cladding with a curable resin that is optically crosslinked by ultraviolet radiation. In practice, to produce the coating of a conductor, the optical fiber is surrounded with a layer of the composition to be crosslinked in the liquid state, after which the material is solidified by exposure to ultraviolet radiation. To effect the crosslinking, photoinitiator elements are generally introduced into the composition to be cured. Thanks to these photoinitiators, the exposure to ultraviolet radiation causes a photochemical curing reaction that leads to curing of the material. Exposure to ultraviolet radiation is usually effected by means of a monochromatic or quasi-monochromatic source of UV radiation, such as a laser, an excimer lamp, or an arc lamp.
The prior art document DE-41 26 860 describes a method of fabricating an optical fiber ribbon in which parallel and previously coated optical fibers are bonded together using an acrylate-based adhesive. It is specified that the crosslinking of the coating of the fiber by UV radiation is usually effected in an inert gas, because the presence of oxygen has the effect of inhibiting the reaction, but the adhesive bonds badly to the plane and fully cur ed surface. The document therefore proposes to effect the crosslinking of the coating in the presence of a defined quantity of oxygen so that the surface of the coating remains microscopically tacky and bonds strongly to the adhesive. The filament passes through the installation at a speed of 30 m/min. The composition is cured first in nitrogen, and then the nitrogen is enriched with progressively increasing amounts of oxygen, in the presence of UV radiation.
According to the above document, in order to obtain incomplete curing, oxygen is used to prevent the acrylate groups from reacting with each other. The fiber drawing speeds used at present in the fabrication of optical fibers are much higher (greater than 300 m/min) than those referred to in the above document. At these speeds a complete reaction is no longer possible.
The coating is conventionally applied in a fiber drawing tower in which the optical fiber travels at a speed in the order of 1 000 m/min to 1 200 m/min and as high as 2 000 m/min. At these speeds, crosslinking is necessarily incomplete at the exit of the fiber drawing installation. However, for reasons of industrial cost effectiveness, it is only possible to effect a single pass at high speed in the installation, and this single pass must lead to complete crosslinking of the coating. The problem that arises here is thus the converse problem of accelerating the crosslinking of the coating composition in the installation and enabling the reaction to continue after leaving the fiber drawing installation.
An object of the present invention is therefore to propose a method of coating an optical fiber with a curable composition that can result in complete crosslinking whilst necessitating only a single pass at high speed in the fiber drawing installation.
Another object of the invention is to propose an optical fiber having a cured coating obtained by the above method and a telecommunication cable including this kind of fiber.